Anal. Chem. 2004, 76, 3395-3416
Inductively Coupled Plasma Mass Spectrometry Diane Beauchemin
Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada Review Contents Conferences Books and Reviews Sample Preparation Sample Introduction Nebulizers Spray Chambers Vapor Generation Electrothermal Vaporization Laser Ablation Speciation Methods Spectroscopic Interferences Sample Pretreatment Computational Approaches Instrumental Approaches Nonspectroscopic Interferences Fundamental Studies Mass Spectrometer Interface Chemometrics Isotope Ratios Isotope Dilution Instrument Performance Literature Cited
3395 3396 3396 3397 3398 3399 3399 3400 3401 3404 3408 3408 3408 3408 3409 3409 3410 3410 3411 3412 3413 3413
Since the last and first fundamental review on inductively coupled plasma mass spectrometry (ICPMS) published in Analytical Chemistry (1), the number of its applications has further increased while the number of fundamental studies has decreased. This trend would lend credence to the belief that ICPMS is now a routine technique. However, its remaining and persisting shortcomings have still not been resolved. The purpose of this paper is to critically review significant developments in the ICPMS field during the period from October 2001 to October 2003 (exclusively). Because it must be limited to a maximum of about 200 references, this review is not meant to be comprehensive but nonetheless includes several references to relevant books and reviews that provide a more comprehensive coverage. To minimize error propagation, all the articles referenced in this review were read (and, hopefully, correctly understood!). The following journals were systematically perused: Analyst, Analytical Chemistry, Analytical Chimica Acta, Applied Spectroscopy, Applied Spectroscopy Reviews, Canadian Journal of Analytical Sciences and Spectroscopy, Critical Reviews in Analytical Chemistry, Analytical and Bioanalytical Chemistry (formerly Fresenius' Journal of Analytical Chemistry), ICP Information Newsletter, Journal of Analytical Atomic Spectrometry, Journal of the American Society for Mass Spectrometry, Microchemical Journal, Spectrochimica Acta Part B, Spectroscopy, Talanta, and Trends in Analytical Chemistry. (The journal that consistently contained, by far, the greatest number of papers on ICPMS was the Journal of Analytical Atomic 10.1021/ac040068n CCC: $27.50 Published on Web 04/30/2004
© 2004 American Chemical Society
Spectrometry.) Nonetheless, this perusal alone led to some 600 references. Since many other journals include ICPMS publications, several people could have a full time job just reading all the ICPMS papers that are published everyday! Needless to say, this made difficult the selection of the most significant or representative papers. Furthermore, this overwhelming amount of information has deepened this author’s belief that it is now essentially impossible for authors to quote all relevant work even with abstracting services. Indeed, these searches are highly dependent on the keywords selected, keywords that may not appear anywhere in the title and abstract if the authors have put a different emphasis on their work. Very thorough and vigilant reviewer are therefore required to review manuscripts. Editors should also ensure that the most knowledgeable people review papers. For example, when a paper discusses a method that is conceptually similar to a previously published one, at least one author of the latter should be asked to act as reviewer in order to verify whether the new method is indeed significantly different from the previous one. CONFERENCES There are so many conferences being held all over the year that, if one could afford it, one could literally spend all one’s time going from one conference to the next! However, if only one meeting can be attended, then one of two conferences should be selected: either the Winter Conference on Plasma Spectrochemistry or the International Conference on Plasma Source Mass Spectrometry. The former, which is held alternately in the United States (even years) and Europe (odd years) at the beginning of each year, was in Fort Lauderdale, FL, in January 2004 and will be in Budapest, Hungary, January 29-February 2, 2005 (see www.conferences.hu/winter2005). The latter, which is held in September on even years at the World Heritage Durham Castle in Durham, England, will again be at the University of Durham, England, September 12-17, 2004 (for further information, contact
[email protected]). Although both conferences are not strictly restricted to ICPMS (for example, other ion sources such as glow discharge are included), ICPMS is by far the most prominent topic. Furthermore, both conferences have no parallel oral sessions; i.e., nothing can be missed. However, this advantage seems to be somewhat eroding at the Winter Conference on Plasma Spectrochemistry when it is held in the United States since manufacturers are now making oral presentations in parallel with the poster sessions. Those who must be at their poster cannot attend the presentations, while other people must select which presentations to attend if they want to also have enough time to look at all the posters. The program is also so packed (typically from 8 a.m. to 6:30 p.m., with barely an hour for lunch if speakers do not go overtime) that it can be an overwhelming and tiring Analytical Chemistry, Vol. 76, No. 12, June 15, 2004 3395
Table 1. Books Related to ICPMS title Elemental Speciation, New Approaches for Trace Element Analysis; Vol. XXXIII of Wilson & Wilson’s Comprehensive Analytical Chemistry Direct Sample Introduction Techniques for Inductively Coupled Plasma Mass Spectrometry; Vol. XXXIV of Wilson & Wilson’s Comprehensive Analytical Chemistry Analytical Methods for Environmental Monitoring Applications of Inorganic Mass Spectrometry; Wiley-Interscience Series on Mass Spectrometry Analytical Atomic Spectrometry with Flames and Plasmas
authors/ editors
publisher
year
reviewer
ref
J. A. Caruso, K. L. Sutton, and K. L. Ackley, Eds
Elsevier
2000
H. Emteborg
2
D. Beauchemin, D. C. Gre´goire, D. Gu ¨ nther, V. Karanassios, J.-M. Mermet, and T. J. Wood F. Taylor, M. Cartwright, R. Ahmad J. R. de Laeter
Elsevier
2000
G. de Loos D. Butcher M. Krachler
3-5
Prentice Hall
2000
J. Feldmann
www.rsc.org/ anlreview
WileyInterscience Wiley-VCH
2001
6, 7
2001
C. L. Wilkins J. S. Becker G. Greenway J. Tyson D. Butcher P. Van Calsteren
11
2001
J. W. Olesik
12
2001
S. Stu ¨ rup G. S. Hall
13, 14
J. A. C. Broekaert
Laser-Ablation-ICPMS in the Earth Sciences
P. Silvester, Ed.
Inductively coupled plasma-mass spectrometry: practices and techniques Plasma Source Mass Spectrometry-The New Millennium (Proceedings of the Durham Conference)
Howard E. Taylor
Mineralogical Association of Canada Academic Press
G. Holland and S. D. Tanner, Eds.
Royal Society of Chemistry
experience. The International Conference on Plasma Source Mass Spectrometry may be more relaxed, although this author has never been able to attend it for financial or bad timing reasons (i.e., it always falls at the beginning of classes that this author teaches!). People who have other interests (such as molecular spectroscopy) in addition to ICPMS may prefer to attend the annual meeting of the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS) where, however, there are many parallel sessions. An alternative, which is growing in popularity, is the International Conference on Analytical Sciences and Spectroscopy (ICASS) that is held annually in Canada. The latter has the same high quality but is smaller in size than FACSS so that there are not as many choices to make in regard to which talk to attend (for this reason, this author personally prefers ICASS). In 2004, ICASS will be in Halifax, Nova Scotia, August 15-19, and several special events are planned (lobster banquet in Natural Science Museum, half-day break to tour Halifax Harbor in amphibious vehicles, etc.). This conference is indeed sponsored by the Spectroscopy Society of Canada, which celebrates its Golden Jubilee in 2004. It will feature a special symposium in honor of Prof. Jean-Michel Mermet, a guru of ICP spectroscopy who will be retiring at the end of 2004. (For more information, see www.smu.ca/ICASS2004.) Later in 2004, FACSS will be in Portland, OR, October 3-7 (see www.facss.org). In 2005, the two will be jointly held in Quebec City, October 9-13. Those who need more information on plasma-related events may consider subscribing to the ICP Information Newsletter, edited by Ramon M. Barnes, which publishes a calendar of plasma-related events in addition to reports on symposia and plasma-related abstracts of papers presented at numerous conferences worldwide. BOOKS AND REVIEWS Table 1 lists detailed reviews written by experts on books that contain at least one chapter on ICPMS. Numerous general review 3396 Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
2001
8-10
articles were also published (reviews on specific topics are referenced in the relevant sections). The series of tutorials on ICPMS that was started during the last review (1) was continued with a focus on mass analyzers (quadrupole (Q) (15), doublefocusing (DF) magnetic sector (16), time of flight (TOF) (17) and collision/reaction cell (18)), detectors (19), peak measurement approaches (20), interferences (21) (both spectroscopic and nonspectroscopic), and even sampling accessories such as laser ablation and flow injection (FI) (22), as well as electrothermal vaporization, desolvation devices, and chromatographic separation approaches (23). The operating principles as well as the advantages and disadvantages of the two different configurations of mass analyzers (i.e., orthogonal vs axial) that are currently used for ICPTOFMS were thoroughly discussed (24). Although both offer clear advantages for complete mass coverage during the detection of transient signals, they do not yet compete in terms of sensitivity and detection limits with ICPMS using scanning mass analyzers. The reason for this is that discrete ion packets, which are required for acceleration in a TOF analyzer, are obtained through a reduction in duty factor (i.e., fraction of ions from the plasma that are extracted into the MS). Several reviews on atomic mass spectrometry (25, 26) and atomic spectroscopy (27) included significant developments in instrumentation and methodology as well as improved understanding of the fundamentals of ICPMS. The use of ICPMS was also prominent in reviews on industrial analysis: metals, chemicals, and advanced materials (28, 29), environmental analysis (3032) and clinical and biological materials, food, and beverages (33, 34). SAMPLE PREPARATION Accuracy in chemical analysis by ICPMS (or any other technique) depends not only on the measurement process but also on all the previous steps including sampling and sample preparation (35). Isotope dilution (ID) is the calibration strategy
that may provide the most accurate results and even serve as a definite method of analysis. However, in addition to requiring two isotopes free of spectroscopic interference per analyte, it requires a good isotopic equilibrium between the isotopic spike and the sample. Only then will it adequately compensate for any artifact during the sample preparation (such as loss of analyte through evaporation). The sample preparation procedure should therefore be selected with this requirement in mind. If speciation analysis is the ultimate goal of the analytical procedure, then every single step should be even more carefully selected to preserve the various species, i.e., prevent their interconversion or degradation. This was clearly demonstrated in a study on edible rice where, following grinding, monomethylarsonic acid and As(V) were found to be unstable under any storage conditions, unless the rice powder had been γ-irradiated (36). Any extraction technique should similarly be selected carefully. For example, microwave-assisted extraction induced degradation reactions of organotin compounds whereas no such degradation was observed using ultrasonic extraction (37). In fact, multiisotope-labeled ID analysis is a powerful tool that, provided isotopically labeled species are available or can be synthesized, allows the optimization of all the steps in a speciation analysis procedure, from the extraction of analytes to their derivatization (for GC separation for example), separation, and detection by ICPMS (37). The use of focused microwave radiation in open-vessel sample preparation strategies, such as extraction and digestion procedures, was thoroughly reviewed (38). Compared to closed-vessel approaches in a microwave oven, focused microwave-assisted strategies are safer and more versatile since they are conducted at atmospheric pressure, the amount of microwave energy can be controlled, and the addition of various reagents can be programmed as part of the procedure. This results in a good control of the reaction(s), which in turn enables the selective extraction of analyte species for speciation analysis. Approaches for reducing the acid concentration in the final digest were also described, in particular acid vapor-phase digestion and the gradual addition of the sample to a small amount of hot reagent (38). These approaches are particularly attractive for ICPMS since they significantly reduce the blanks, which are too often limiting to the quantitation limit. However, microwave digestion is not suitable for all sample types. For instance, it was inadequate for the determination of Cr in silicate materials containing refractory Cr-containing minerals (such as in sediments) (39). A 50% recovery was, at the most, achieved even when microwave digestion with HNO3, HF, and HClO4 was followed by a reflux with concentrated HClO4. An adequate alternative was digestion in a sealed Carius tube using only HNO3 and HCl because both high pressure (in excess of 100 atm) and temperature (up to 400°C) could be achieved (39). On the other hand, the handling of these tubes is not as convenient as screwing and unscrewing the caps of microwave digestion vessels, since they must each be sealed in a flame and must be handled and vented extremely carefully when they are pressurized. Indeed, since all the carbon is converted into CO2, additional pressure is generated in the tube, which may represent an explosion hazard. Carius tube digestion is therefore best suited for samples with low carbon content.
Electroerosion in aqueous solution, where a high current was applied between two conducting sample rods in a conductive (0.01 M NaCl) solution was proposed as a quick alternative to acid digestion for the analysis of conductive samples (such as metals) (40). For instance, only 20 s at high current (10 A cm-2) was sufficient to remove a significant amount of sample (e.g., 10 mg). Of course, the required erosion time will depend on analyte concentration. In any case, although electroerosion could be carried out on-line with ICPMS, analyte signal precision was significantly improved if the colloidal suspension resulting from electroerosion was dissolved in HNO3 prior to its nebulization (40). A simple, yet innovative approach was developed by Beauchemin et al. where sample treatment was performed on-line with ICPMS to gain information on the mobility and fractionation of elements in soil samples (41). The approach involves monitoring the leaching profiles of both trace and matrix elements while reagents (water and 1, 10, and 30% HNO3), are sequentially pumped, either continuously or using flow injection, through a microcolumn of soil. Through comparison of the profiles of analytes and matrix elements, it provides real-time data on the phases that are breaking down. In the case of elements whose isotopic distribution varies in nature, confirmation of different sources can be obtained through the continuous monitoring of their isotopic ratios. On the other hand, because of the complexity of the resulting mass spectra, high-resolution MS is required to resolve spectroscopic interferences on several analytes. SAMPLE INTRODUCTION The selection of a sample introduction system depends on a number of parameters such as the amount of sample available, the type of matrix, etc. What is often not realized is that measurement precision may be limited by the sample introduction step. Noise power spectra can be used to characterize the types of noise associated with a given system and determine the best measurement frequency for a given application (42). For example, although a direct-injection high-efficiency nebulizer (DIHEN) provided a 750 GW/cm2) (106). At irradiances between 500 and 750 GW/cm2, a better accuracy was obtained by averaging the signals obtained from the first two laser shots. In fact, increasing the laser irradiance decreased the particle size, which reduced the magnitude and frequency of spikes on the transient signal. This study clearly demonstrated that fractionation increased with the number of laser pulses, i.e., with the ablation time, and that the initial few pulses allowed a representative analysis of a glass sample. It will be interesting to see whether this observation can be generalized to other matrixes. Alternatively, a femtosecond laser providing100-fs laser pulses at 800 nm, was reported to similarly allow a representative sampling of glass samples when the laser fluence was 1.5-3.2 J/cm2 (107). In contrast to when nanosecond laser pulses are used, no raised rim was observed around the craters, which indicates either reduced plasma interaction or a change in the primary ejection mechanism. Fractionation was observed at higher fluence. Multiple internal standardization for each analyte was demonstrated to yield more precise results than those obtained using a single internal standard (108). These multiple internal standards, which can simply be different isotopes of the same element, allow the extraction of a drift pattern from the concurrent measurement noise that is otherwise propagated into the signal of normalized elements. A weighing of signal precision is also done to emphasize the more precise measurements, which in turn improves the estimation of the normalization factor. A bilinear model was found to adequately represent the measurements since there was a measurement-dependent pattern and an internal standard-dependent scale. This model was fitted to the measurements by minimizing a least-squares cost function. The value of this cost function provides a good indication of artifacts in the measurements and Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
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therefore provides internal quality control. In any case, the variance on the corrected intensity versus time profiles was observed to be inversely proportional to the number of internal standards; i.e., the RSD decreased proportionally to the square root of the number of internal standards (108). Therefore, two internal standards could improve precision by nearly 30%, three by 42%, four up to 50%, etc. However, this approach will not be as useful for fractionation-prone elements since the stochastic noise on these elements will remain. A completely different approach was taken by Kosˇler et al., who quantified the laser-induced elemental fractionation by doing a linear regression of the ratio of the analyte signal over that of a reference element as a function of time in time-resolved analysis mode (109). This approach separates the elemental fractionation (quantified by the slope) from the analytical noise. It assumes that the uncertainty in the number of laser pulses is negligible and that the uncertainty in the ratio is constant. This means that a direct comparison between different laser ablation experiments is only possible if they have been performed under near-identical conditions. In any case, this approach revealed that fractionation was dependent on the amount of oxygen in the ablation cell, whether present as a trace in the carrier gas or released by the sample during ablation. Close matrix-matching is therefore required for the quantitative analysis of samples containing oxygen (such as oxides and silicates) by external calibration. A powerful alternative calibration strategy is ID, where the ideal internal standard is used for each analyte since an enriched isotope of each analyte is added to the sample. This was implemented for powder samples, where the isotopic spike solution was simply added to the sample and a homogeneous suspension was obtained by shaking (110). In the case of organic samples, a concurrent addition of methanol may be required to completely moisten the powder. The suspension was then dried at 60 °C, homogenized with a Teflon pestle, and pressed into 20-mm-diameter pellets. Although a preliminary analysis of the sample is required to determine the amount of isotopic spike to be added, this may be a small price to pay for results of high accuracy. However, the approach is not applicable to monoisotopic elements who do not possess long-lived radioactive isotope (such as As). Furthermore, erroneous results were obtained for some elements, such as Cr and Fe, in some materials, which will require further investigation. Table 4 gives a few examples of quantitative methods. In fact, there has been so much activity in using LA for spatial profiling and semiquantitative and quantitative analysis that a separate fundamental review could probably be devoted to LA-ICPMS! Speciation Methods. Szpunar et al. discussed various strategies involving techniques hyphenated to ICPMS for the analysis of trace element speciation in biological systems (119). Their review covered from the simplest case, i.e., where a separation technique (such as chromatography, capillary electrophoresis (CE), or even gel electrophoresis) is coupled to ICPMS through the nebulizer or LA (for gel electrophoresis) to multidimensional separation and detection approaches. The latter are indeed required to cope with the complexity of many biological matrixes. Nonetheless, because of its great sensitivity and selectivity, trace element speciation by ICPMS has great potential for proteomics (120). It indeed enables studies on metal-biomolecule associations/dissociations, which in turn provides information on the 3404
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mechanisms of biological action of metals (or semimetals) in a biological organism. Another review focused on chiral trace element speciation in biological samples, which involves the determination of enantiomeric forms or species of a given element and, more specifically, on the chiral speciation of Se in selenoamino acids, by either highperformance liquid chromatography (HPLC)-ICPMS or gas chromatography (GC)-ICPMS (121). In fact, because Se has a major nutritional role as well as cancer chemopreventative properties, research on the development of hyphenated techniques for its speciation has been so active that a review was written on it by Uden (122). Although various separation techniques have been used (HPLC, GC, CZE) in hyphenation with ICPMS, HPLCICPMS and CZE-ICPMS are sufficiently sensitive for the selective detection of Se-containing compounds in many samples. An extensive review was devoted to the speciation analysis of As (123). It covers from sample preparation and species conservation through separation methods (especially liquid chromatography and CE) and detection methods (i.e., direct monitoring by ICPMS or by hydride generation coupled with ICPMS). The use of ID for speciation studies by ICPMS was reviewed, including uncertainty contributions to species-specific ID analysis (124). The issue of extracting an isotope ratio from a chromatographic peak was also discussed, along with the advantages and disadvantages of peak integration versus averaging point-to-point ratios taken at the top of the peak. While peak integration can compensate for fractionation and spectral skew, and should therefore provide ratios of higher accuracy than the point-to-point approach in the presence of these artifacts, the latter should provide the ratios of highest precision and, in fact, allow an estimation of the precision from a single peak. This, in turn, significantly decreases the sample volume and time required for an analysis. As expected, species-specific ID (using point-to-point ratios from the top of the chromatographic peak) yielded more precise results with MC-ICPMS than a sequential scanning instrument (125). In fact, a major contribution to the expanded uncertainty then came from the uncertainties in the natural isotopic abundances. Therefore, a refinement of the isotopic characterization of several elements (i.e., IUPAC data) will be required to facilitate the wide use of ID. When species-specific enriched spikes are not available, then species-unspecific ID can be carried out to ensure the most accurate and precise determination of the analyte species. This can be simply achieved by automatically and continuously merging a stream of enriched spike with the effluent of the chromatographic separation approach prior to its introduction into ICPMS (126). However, to obtain the most accurate result, the spike mass flow should be determined gravimetrically as it was shown that reverse ID, which involves injecting a standard solution with natural isotopic abundance into the separation system, could yield biased results. In any case, all the hyphenated speciation techniques rely on retention times and the availability of standards for compound identification. Unless species-specific ID analysis is performed, confirmation of any identification must be carried out by other techniques such as hyphenated molecular characterization by MS or NMR. A few examples of speciation methods that have been applied to the quantitation of selected species are listed in Table
Table 4. Selected LA Quantitative Methods analytes
sample matrix
sample preparation
8-13 elements
glass and steel SRMs
38 elements
ice core
8.5 cm long, 3.5 cm wide, 1 cm high sample fixed on Teflon carrier using frozen water drops
16 elements (Pb incl)
automotive paint
10 elements
pure alkaline earth fluoride powders
40 elements
glass
11 elements
lubricating oils
homogeneization in ultrasonic bath at 50 °C; 0.3-mL sample deposited in polyethylene vial
Cr, Cu, Zn, Cd, Pb
soil SRMs
La, Sr, Mn, Co
compositionally graded perovskite layers
slurry of sample with 10% v/v Triton X-100 and standard spikes dried at 100°C; pressed into pellets pressed perovskite powders sintered in the form of disks
Ni
rat tissue
detection method (MS type)
calibration strategy
features
ref
Finnigan MAT Element 1 (DF) with shielded torch; R ) 4000; LA effluent merged with pre-evaporated aerosol from microconcentric nebulizer/spray chamber PE-SCIEX ELAN 6000 (Q); patented cryogenic LA chamber
2-point EC with solutions through nebulizer; normalization for mass transport using on-line microbalance
no matrixmatching or prior knowledge of sample composition required; for analytes free of fractionation
96
EC with frozen standard solutions; 17OH internal standard
111
sample (3-4 paint layers on steel) cut into 2-cm squares
HP-4500 (Q) with shield torch
EC with SRMs or in-house matrix matched standards
sample mixed with multielement enriched-isotope spike solution, dried and pressed into pellets
Finnigan MAT Element 2 (DF); R ) 300 or 4000
ID
PE-SCIEX ELAN 6000 (Q); high-power 266-nm Nd:YAG laser with homogenizing optics (i.e., energy distribution is independent of crater diameter) Leco Renaissance (TOF); ablation cell volume reduced from 55 to 19 mL with PTFE inserts; impactor added to remove big droplets PE-SCIEX ELAN 6100 (Q with DRC)
EC with internal standardization using major elements
method validated by accurate analysis of frozen SRMs; provides temporal variation by spatial profiling of ice core fingerprinting of paints through time-resolved analysis of each paint layer simple implementation of a definitive method of analysis; lower detection limit than after wet dissolution flexible adjustment of spatial resolution for each individual analysis; reduced elemental fractionation
direct multielement analysis of complex organic matrix; cool plasma conditions used for Fe and Cr to reduce ArO+ and ArC+ interferences quicker sample preparation than acid digestion; DRC required to eliminate interferences on Cr analyte concentration profiles with µm spatial resolution
115
direct analysis of soft tissue; spatial profiling of Ni concn vs time
118
tissue embedded in paraffin
PE-SCIEX ELAN 6000 (Q)
Finnigan MAT Element 2 (DF);
5. Some speciation methods are also included in Tables 2 and 3. This is an other area of intense activity. Gas Chromatography. Bouyssiere et al. comprehensively reviewed the use of GC coupled to ICPMS for speciation analysis (136). Critical aspects were covered such as the design of the interface between GC and ICPMS and sample preparation methods, including available derivatization reagents and advanced extraction techniques (such as solid-phase microextraction and
EC with oil standards or aq solutions nebulized and desolvated prior to passage through LA cell while blank oil is ablated MSA or ID
quantitation using proportions of Coand Mn-perovskite (determined from Mn and Co signals), by weighed summation of elemental concn in individual perovskite EC with glass SRM using 24Mg internal std
112
113
114
116
117
capillary purge and trap). Another review focused on the use of isotopically labeled organometallic species to either detect errors during sample preparation or for quantification by isotope dilution (137). In fact, spikes containing different species labeled with different isotopes are highly recommended for validation purposes since they allow the detection and correction for species interconversion or degradation in every step of the procedure, including the initial solid-liquid extraction. Furthermore, these Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
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Table 5. Selected Speciation Methods analyte species
sample matrix
total homocysteine
human serum
phosphate
seawater
sample pretreatment reduction with NaBH4; separation on Dowex 5WX2200 resin; evaporation to dryness; conversion to N-trifluoroacetylO-isopropyl derivatives
Ge, Sb, As
seawater, FI-HG; cryogenic freshwater trapping of hydrides; SRM cryofocusing by lowering/raising the trap mono-, di-, marine microwave-assisted and sediment extraction in tributyltins SRM acetic acid; pH of supernatant adjusted to 5-6 mono-, di-, marine microwave-assisted and sediment extraction; tributyltins (TBT) SRM ethylation of analytes; extraction into isooctane
separation method
ICPMS brand (MS type)
calibration strategy
Finnigan MAT EC with Element (DF, ethionine as R ) 3000); internal std guard electrode; 32 + using S
quantitative determination of total homocysteine in human serum
127
ion-exchange chromatography; 50 mM HCl eluent
PE-SCIEX ELAN 6000 (Q); using 31P+ PE-SCIEX ELAN 6000 (Q)
MSA
direct determination of phosphate (no extraction) multielement speciation using aqueous standards
128
PE-SCIEX ELAN 6000 (Q)
MSA or ID (for TBT)
130
PE-SCIEX ELAN 6000 (Q)
MSA or ID (for TBT)
rapid sample preparation; simple coupling of HPLC to ICP far better sensitivity and chromatographic resolution than using HPLC
Agilent 7500c (Q); shieldtorch; octopole reaction cell with Xe
EC with S and Cd compds; Ge internal std in makeup solution postcolumn speciesunspecific ID
multielement speciation using speciesunspecific elementcontaining compounds fast, accurate determination of analytes bound to fish cytosols of unknown composition and structure simultaneous speciation of Hg and Sn in biological samples
131
heating of the SE-30 5% Chromosorb W-HP 80-100mesh cryogenic trap cation-exchange LC; 0.16 M ammonium citrate at pH 4.8 in 60:40 MeOH/H2O temp-programmed GC with DB-1 column and He carriergas; homemade GC-ICPMS interface CE with TrisHNO3 20 mM, pH 7.0-7.4 buffer; modified micronebulizer interface to ICPMS
EC with aqueous stds
metallothioneins (MT)
Cu, Zn, Cd bound to MT
carp and eel cytosols
20-fold dilution of cytosol sample
size exclusion HPLC; 30 mM Tris-HCl pH 7.4 mobile phase
Leco Renaissance (TOF)
MeHg, mono, di, and tributyltins
oyster tissue SRM
5-min open-focused microwave extraction in TMAHa; derivatization through ethylation
PE-SCIEX ELAN 6000 (Q)
EC
MeHg, tributyltin
oyster tissue SRM
PE-SCIEX ELAN 6000 (Q)
speciesspecific ID using monoMe201Hg and tributyl117Sn
inorganic Hg, MeHg, EtHg
mouse tissues, biological SRMs
5-min openfocused microwave extraction and spike stabilization in TMAH; derivatization through ethylation tissue mixed with spikes in TMAH; adjusted to pH 9; extraction into toluene with DDTCb; derivatization with Grignard reagent
temp-programmed capillary GC with MXT Silcosteel column and He carrier gas; homemade GC-ICPMS interface temp-programmed capillary GC with MXT Silcosteel column and He carrier gas; homemade GC-ICPMS interface temp-programmed capillary GC with SPB-1 column and He carrier gas; homemade GC-ICPMS interface
Agilent 7500a (Q); 3 mL/min O2 in Ar auxiliary gas against soot deposits
speciesspecific ID using Me200Hg+, Et199Hg+, 201Hg2+
arsenic species
edible algae powder
a
ref
temp-programmed GC with HP-5 capillary column and He carrier gas; homemade GC-ICPMS interface
S, Cd
repeated extraction into MeOH/H2O; evaporation to dryness; dissolution of residue in water
features
off-line size PE-SCIEX exclusion; AsELAN containing 6000 (Q) fractions pooled; gradient anion exchange and then RP HPLC; some fractions submitted to isocratic cation exchange
TMAH, tetramethylammonium hydroxide. b DDTC, diethyldithiocarbamate.
3406 Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
MSA
129
130
132
133
simultaneous 133 determination of MeHg and tributylSn in biological samples; better precision than by EC species-specific 134 ID allowed accurate analysis despite some EtHg decomposition during sample preparation multidimensional 135 HPLC allows mapping of naturally present As species in biological materials
species mixtures labeled with different isotopes are also invaluable to study extraction procedures since they not only provide information on the extent of degradation reactions but also allow their optimization such that quantitative extraction is achieved (138). The very sensitive ICPMS detection can even be extended to the quantitative determination of organic compounds (139). For example, n-alcohols (such as butanol, pentanol, hexanol, and heptanol) could be determined by temperature-programmed GC with a low-bleeding capillary column after their conversion to trimethylsilyl ethers by reaction with N-methyl-N-trimethylsilyl trifluoracetamide in isooctane. Either external or internal calibrations could be used for structure-independent quantification. Furthermore, under the GC conditions used, the most abundant Si isotope (28Si) was free of spectroscopic interference. Therefore, this method is applicable to all ICPMS instruments, i.e., does not require high-mass resolution. A GC-ICPMS interface was designed to facilitate the speciation of semivolatile analytes while allowing the injection of a large volume of solvent (140). It involves a dilution of the GC column effluent with preheated Ar makeup gas (containing a small amount of oxygen to prevent soot deposition on the MS interface) within the GC oven before it flows through a heated steel transfer line that replaces the torch injector. This dilution reduces the condensation temperature of the analytes, thereby facilitating their transfer into the ICP. Since the solvent peak is concurrently eliminated by a periodic flow reversal induced in the transfer line by a vacuum pump, which completely prevents problems of soot deposition from the solvent, larger volumes can be injected to compensate for the dilution. The reproducibility based on 10 replicate 20-µL injections was not only improved but internal standardization using Xe gas was no longer required (as it degraded precision). On the other hand, the detection limit was somewhat higher than that obtained with a conventional interface (i.e., where the makeup gas is added through the torch injector and does not thoroughly mix with the GC effluent). The addition of oxygen to the dry plasma that is typically required to prevent carbon deposition on the MS interface must be optimized carefully since excess O2 will degrade the MS interface while not enough will result in clogging by soot deposition. This problem was completely resolved using a completely different GC-ICPMS interface that was developed by Donard’s group (141). It involves a resistively heated steel transfer capillary, which is inserted into the torch injector through a tee to which is connected a cyclonic spray chamber with a Meinhard concentric nebulizer. The latter allowed continuous nebulization of a 10 µg/L Tl solution, which was used for instrument optimization and mass bias correction (through the measurement of 203Tl and 205Tl) of GC-ICPMS measurement. The normal oxygen content of this aqueous solution was sufficient to prevent carbon deposition on the cones. Nonetheless, mixed-gas plasmas, which as will be discussed later, can decrease nonspectroscopic interferences and can also be used to enhance the detection of elements with a high first ionization potential (IP). Indeed, an addition of nitrogen to the makeup Ar gas of the carrier gas from a gas chromatograph was shown to enhance the sensitivity of phosphorus by over 1 order of magnitude (142). However, the concurrent increase in back-
ground, which was attributed to increased formation of 15N16O+, was compensated without a sacrifice in sensitivity, by using a collision cell with He and kinetic energy discrimination (KED). Similarly, adding either O2 or N2 to the Ar makeup gas enhanced the sensitivity for Se species, with N2 providing the greatest enhancement (143). Although this addition induced spectroscopic interferences in low-resolution MS, 77Se was unaffected and could be used for quantitation. Liquid Chromatography. General aspects of the coupling of liquid chromatography (LC) with ICPMS for speciation analysis were thoroughly reviewed (144). Problems resulting from the interfacing (such as the effect of salts and organic solvents in eluents on analyte signal, and the dead volume of the interface) were also discussed as well as means of circumventing them (such as the combination of micro-LC with a micronebulizer). Nonetheless, applications of LC-ICPMS are increasing, especially for the speciation analysis of As, Cd, Se, I, Sn, and Pt (145). Another review focused on the instrumentation and performance of systems used for the determination of compounds containing nonmetals (C, S, P, and halogens) by HPLC with detection by either ICP optical emission spectrometry or ICPMS (146). Hybrid techniques with ICPMS, which are available for multimetal speciation in metallothioneins and metallothionein-like proteins, were also reviewed, including the advantages and limitations of the separation technique (such as size exclusion, anion-exchange, or reversed-phase HPLC) and the ICPMS detector (Q, DF, or TOF) (147). Since biological systems are quite complex, the sequential use of different separation techniques is recommended to chromatographically resolve new metal-biomolecules prior to their detection by ICPMS. Capillary Electrophoresis. In addition to HPLC-ICPMS, the characterization and quantification of metallothionein isoforms can also be carried out by CE-ICPMS (148). As pointed out above, the sequential application of different separation steps, such as size exclusion to fractionate a cytosol followed by CE, can improve the separation of the metal-biomolecules if measures are taken to prevent the concurrent degeneration of metallothioneins. In any case, the interface between CE and ICPMS is critical because the smallest suction effect from the nebulizer would degrade the separation of the different MT isoforms and subisoforms, which elute very close to each other. Furthermore, the interface should preserve sensitivity since the metal content of MTs is very low in living organisms (149). The MicroMist micronebulizer was reported to provide better detection limits than the HEN, especially when used with a double-focusing ICPMS instrument. Furthermore, the high sensitivity of the latter when operated in low-mass resolution (i.e., R ) 300) provided detection limits that were a sixth of those obtained using the same interface with a quadrupole-based instrument. SPECTROSCOPIC INTERFERENCES Spectroscopic interferences arising from matrix components are inherently eliminated by sample introduction strategies that involve a separation of the analytes from the sample matrix, such as vapor generation and ETV. When such systems are not available or applicable, other approaches can be used, either through sample pretreatment prior to analysis or through appropriate ICPMS operating conditions. Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
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Sample Pretreatment. An extensive review focused on SPE of trace elements, which involves partitioning of analytes between a liquid sample and a solid sorbent phase (150). It allows selective analyte extraction as well as preconcentration if the extracted analytes are eluted into a smaller volume of eluent than that of sample used. This approach is very popular with ICPMS since it allows a removal of problematic matrixes that would otherwise induce matrix effects and spectroscopic interferences or result in clogging problems. Although microcolumns, disks, or disposable cartridges and syringe barrels can be used off-line, the use of microcolumns is preferred on-line. A step-by-step development guide is provided to help the selection of the solid sorbent and optimization of the SPE procedure (150). In fact, several preconcentration approaches have been implemented on-line to ICPMS using flow injection methodologies (151). Although liquid-liquid extraction has been used, liquid-solid separation methods are the most widely used, such as SPE on microcolumns, precipitation and coprecipitation, and sorption in a knotted reactor. On-line analyte separation and preconcentration can indeed be carried out through sorption of hydrophobic organometallic complexes onto the wall of a knotted reactor, washing of remaining sample matrix, and elution of the analyte (152). This approach can even be used for the selective determination of specific analyte species. Field flow fractionation (FFF) was proposed as an alternative on-line technique for simultaneous matrix removal and 50-1400fold analyte preconcentration (153). A high molecular weight polymer that selectively forms strong chelating complexes with the analytes allowed matrix removal by permeation through a membrane while the complexed trace analytes remained in the FFF channel. They could then be focused through adjustment of the backward and forward flow rates and directed to the nebulizer. Sampling of the vapor above a solid sample by SPME was proposed as a quick screening technique for the determination of volatile and semivolatile analytes directly from the solid (154). Indeed, no pretreatment of the solid sample is needed other than placing it in a sealed vial that is heated on a hot plate during SPME sampling. Quick thermal desorption of the analytes is then required, since some semivolatile species may be unstable. Multielement detection of such transient signal is then best achieved using instruments with TOF mass analyzer. The method is, however, semiquantitative at best since calibration is then a problem. Computational Approaches. When it is preferable to avoid sample pretreatment to minimize contamination, deconvolution techniques can be considered. For example, a Bayesian spectral deconvolution employing the massive inference algorithm was demonstrated to be a powerful method for the analysis of mass spectra (155). However, since this algorithm infers species contributions from their isotopic patterns, it is most useful for elements with several isotopes. It requires a list of possible components that significantly contributed to the selected mass region, the raw mass spectrum (corrected for mass discrimination using standard solutions) of the sample, and the standard deviation at each mass. The latter is needed because the goodness of fit of the predicted mass spectrum with the experimental one involves taking the difference between them and scaling it to the observed standard deviation. In any case, the approach may be particularly valuable for techniques where no blank is available (such as LA) since it does not require blank subtraction. 3408
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In an attempt to predict the extent of spectroscopic interference from doubly charged ions, partition functions and reduced thermodynamic potentials of atoms, singly and doubly charged ions of elements with low second ionization potentials were calculated (156). These enabled the calculation of M2+/M+ using the Saha equation and a thermodynamic simulation of an Ar plasma containing analyte atoms and water. As a result, a linear relationship was found between log(M2+/M+) and the second ionization potential of the analyte M. In theory, this relationship should allow the prediction and correction of an M2+ interference from a measured M+ signal. Unfortunately, the coefficients of this relationship are dependent on plasma temperature, which is not known precisely and is not readily measured. Instrumental Approaches. Using a cold plasma will reduce spectroscopic interferences arising from the plasma (Ar+, Ar2+, etc.). However, such a plasma, which typically results from a decrease in power, an increase in aerosol carrier gas flow rate, or both, effectively reduces the residence time in the plasma and, hence, its robustness. As a result, matrix-induced interferences (both spectroscopic and nonspectroscopic) can be expected to worsen significantly compared to those observed in a normal ICP. The analysis of samples with a complex matrix therefore requires a preliminary removal of matrix component. Nonetheless, the accurate analysis of oyster tissues and human serum was performed using a cold plasma in combination with a cationexchange separation for complete removal of Na, Mg, Ca, and K and isotope dilution analysis to compensate for any remaining nonspectroscopic interference and incomplete recovery from the separation procedure (157). In fact, matrix separation with simultaneous analyte preconcentration can be performed on-line with ICPMS, such as the determination of Fe in mineral water where Fe was separated and preconcentrated through selective retention on a microcolumn of desferroxiamine immobilized in sol-gel prior to its elution into a cold plasma for sensitive determination using 56Fe (158). Replacing Ar by Ne is an expensive measure that was taken to reduce the huge 63Cu40Ar and 65Cu40Ar interferences on 103Rh and 105Pd when a Cu2S sample was analyzed by LA or the spectroscopic interferences of 58Ni40Ar and 60Ni40Ar on 98Ru and 100Ru when a NiS sample was ablated (159). A concurrent sacrifice in sensitivity, however, resulted. Nonetheless, despite the high cost of Ne, this approach may be a viable alternative to the purchase of a new ICPMS instrument (i.e., with collision/reaction cell or providing high-mass resolution) when the need to resolve argide-based interferences is only punctual. This would be the case during the direct analysis of a few complex samples (such as blood, minerals, and rocks). An impressive tutorial on how reaction and collision cells can be used to reduce spectroscopic interferences in ICPMS includes the fundamentals of ion collision and reaction (including thermochemistry, energy transfer, and reaction kinetics) (160). Moreover, the design and operation of these cells (including the selection of the collision or reaction gas, as well as possible secondary, sequential reactions, which may or not be advantageous) are also discussed in detail. In fact, collision cells were emphasized as a most significant analytical development for the application of ICPMS in materials science (95). Olesik and Jones compared quadrupole ICPMS with a DRC and sector field ICPMS
for the resolution of spectroscopic interferences (161). Several interferences are readily resolved by simply increasing the resolution with sector field ICPMS, albeit at the expense of sensitivity. On the other hand, no sacrifice in sensitivity but more optimization is required with a DRC (or any collision/reaction cell). Indeed, not only must a suitable reaction gas be selected but the operating conditions of the cell must also be adjusted in a fairly empirical manner as opposed to selecting the appropriate mass resolution setting following a quick calculation of the resolution needed. The introduction of water along with He in a collision cell was found to significantly alter the cell chemistry in a beneficial way since a great reduction of argide polyatomic ions resulted with only a concurrent 30% loss of analyte signal and without new species being formed (162). The effect of moisture was not significant when H2 was used as the collision gas, presumably because this reactive gas then dominated the cell chemistry. However, a separate study showed that the signal intensities of ions that could react with the impurity but not with H2 were affected by the presence of such an impurity (163). The same study also showed that collisions with He reduced the ion kinetic energy of polyatomic ions (that did not react with the collision gas or its impurities) more than that of monatomic ions as a result of the larger collision cross sections of the former. In combination with KED, this could, for example, be used to decrease the oxide ratio. Although the selection of a collision or reaction gas may be relatively straightforward for the determination of a specific analyte, such selection is not as obvious when multielement analysis is to be performed under identical conditions. Indeed, highly reactive gases, such as NH3 and N2O, have been shown to improve the detection limits for some elements while drastically degrading those for others (164). That is why the majority of applications of this technology focus on the determination of selected elements only. Fortunately, a mixture of He and H2 was found to minimize the spectroscopic interference of Ar2+ on 80Se+ and ArO+ on 56Fe+ while not affecting or improving the detection limits for analytes free of spectroscopic interference. In fact, spectroscopic interferences can be reduced using a collision cell through KED, ion kinetic energy effect (IKEE), or their combination (165). Because the collision cell acts as a guided path ion-molecule reactor, the input energy of the ions will affect ion-molecule reactions. The IKEE therefore involves a change in collision cell reactivity through a control of input ion energy, which is achieved by varying the collision cell multipole pole bias. This, in turn, changes the potential difference between the plasma and the hexapole since the plasma offset potential is constant for a given set of operating conditions. Conditions that maximize exothermic reaction rates (such as argide ion removal) were found to be similar to those maximizing analyte transmission but favoring the unwanted formation of oxides. However, the latter can be reduced using KED, which involves the exclusion of slow ions formed in the cell from the mass analyzer through the application of a retarding potential or potential barrier between the collision cell and the mass analyzer. Nonetheless, compromise conditions must be selected. Partial overlap from tailing of an intense adjacent peak could also be alleviated using a collision cell filled with He, which was
found to increase isotopic abundance sensitivity more efficiently than increasing the resolution of the quadrupole mass analyzer (166). Indeed, collisions with gas atoms moderated the ion beam, i.e., decreased the ion kinetic energy, which in addition to improving ion focusing also improved abundance sensitivity, reducing peak tailing of intense peaks by up to 3 orders of magnitude, depending on mass, with a concurrent 10% increase in mass resolution. NONSPECTROSCOPIC INTERFERENCES Fundamental Studies. The complexity of nonspectroscopic interferences could really be assessed in the process of developing a model that might predict them from concurrent changes in nonanalyte signals (167). Despite the prediction models including contributions from 19 nonanalyte signals, the prediction was at best 80% accurate, the accuracy of the prediction decreasing if the matrix concentrations differed from those used to train the model. Nonetheless, this study revealed that, in several cases, the matrix effect by a given matrix on nonanalyte signals was different in the presence of analytes, and a matrix blank could not be used to totally predict the effect on samples. In any case, the 12C+/ 15N+ ratio was identified as a key prediction factor. Axial profiling of the distribution of the oxide fraction MO+/ (M+ + MO+) along the central axis of the ICP can help decipher nonspectroscopic interferences occurring within the ICP from those originating within the sampling interface and mass spectrometer (168). For example, the entire LaO+/(La+ + LaO+) profile shifted to lower sampling depth in the presence of 0.02 M K or Cs, indicating that these matrixes induced earlier desolvation. On the other hand, 0.02 M Na shifted only the lower depth portion of the same profile to a lower sampling position. This would suggest that the Na-induced suppression arose from a shift in atom-ion equilibrium since an increase in the concentration of electrons from Na ionization is only expected to occur low in the plasma. Radial profiling was also shown to be a valuable tool to get information on analyte ionization processes in the ICP and how they may be affected by concomitant elements. For instance, a bimodal radial distribution, which is similar to that observed for the electron number density in the ICP (169), was observed for Ar+ and Ar-containing polyatomic ions in both a normal ICP (170) and a cold plasma (171). However, the distance between the two peaks for the latter was about twice that observed in an ICP under typical operating conditions, thereby confirming the broadening of the central channel that is physically visible. Nonetheless, the similar radial distribution observed for Ar-containing polyatomic ions and Ar+ indicates that the former may have Ar+ as a precursor ion and are formed by its collision with neutral species (Ar, O, etc.). This is further supported by the observation of the Ar+ maximums further out than those for Ar2+, which suggests that Ar2+ might have been formed from the collision of neutral Ar from the central channel with Ar+ from the toroidal zone. Certainly, the density of Ar2+ was calculated to be significant in that region of the plasma (169). Nonetheless, the completely different analyte ion radial distributions, which are all bell-shaped and centered on the central channel, indicate that charge transfer from Ar+ is not their dominant ionization mechanism in the ICP. This bellshaped distribution is in agreement with radial measurements of Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
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analyte ion number densities by laser-induced saturated ionic fluorescence (172). Bimodal radial distributions were also observed in a cold plasma for CO+ and NO+, with NO+ also exhibiting a peak in the center of the plasma (171). More interestingly, not only was the distribution of NO+ in the central channel similar to those from analyte ions, but it was suppressed by the matrix to the same extent as the analyte signal, a greater suppression being observed from matrix elements with lower IP. This suggests that a contributing ionization process in the cold plasma is charge transfer with NO+. In fact, if charge transfer with NO+ is the dominant ionization mechanism in a cold plasma, then only analytes with an IP lower than that of NO (9.26 eV) would be significantly ionized. In any case, a relationship was observed between analyte signal and enthalpy of vaporization for non-oxideforming low-IP elements, indicating that the plasma is not hot enough to even fully vaporize all analyte species. This explains why, although the central channel in a cold plasma was twice as wide as in a conventional plasma, the analyte ion distribution was about a third of the peak width of the analyte profile observed in a normal ICP (170). Adding nitrogen to the aerosol carrier gas flow gave an ICP that physically resembled a cold plasma, i.e., with a wider and more diffuse central channel than in a normal Ar ICP.171 However, more intense background ions and a more pronounced NO+ peak were observed in the center of the mixed-gas plasma compared to a cold plasma. Furthermore, only analytes with lower IPs than that of NO had radial profiles similar to that of NO+, which suggests a charge-transfer ionization mechanism between NO+ and analytes (171). Moreover, NO+ signal was suppressed to the same extent as those from these analyte ions in the presence of Na or K. However, the extent of suppression was significantly reduced compared to that observed in a cold plasma. This is commensurate with the observation of a stronger relationship between analyte signal and IP than with enthalpy of vaporization, which indicates that the mixed-gas plasma was better able to vaporize elements than the cold plasma, hence its characterization as “lukewarm” (171). In contrast, an addition of nitrogen to the outer plasma gas drastically shrank the plasma, which effectively moved the IRZ away from the sampling cone and resulted in much lower analyte signal intensities than in an Ar plasma under otherwise identical operating conditions (170). Sensitivity in such a mixed-gas plasma could be improved by reoptimizing either the aerosol carrier gas flow rate (which would be higher) or the sampling depth (which would be smaller). On the other hand, sampling ions from such a mixed-gas plasma at a greater sampling depth than that for optimal sensitivity was shown to provide very robust conditions. Indeed, when sampling 2 mm higher in the ICP than for maximum sensitivity, the matrix-induced signal suppression from seawater was completely eliminated, which enabled its direct quantitative analysis using a simple external calibration without matrix matching (173). In any case, the addition of N2 transformed the bell-shaped analyte radial profiles into bimodal ones that were similar in shape to the flattened and broadened profiles of Ar2+ and Ar-containing polyatomic ions. This suggests that charge transfer from Ar+ may be a predominant ionization mechanism in such a mixed-gas 3410
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plasma (170). In contrast to the lukewarm mixed-gas plasma described in the previous paragraph, there was no correlation between analyte signal and IP. Clearly, the location where a foreign gas is added seems to affect processes in the ICP. Mass Spectrometer Interface. To prevent soot buildup on the sampling interface, which in turn leads to signal suppression, an Ar-O2 mixed-gas plasma was required for the direct analysis of organic samples, whether minute sample amounts were introduced using a DIHEN (174) or a modified DIN (52) (as discussed earlier), or ultrasonic nebulization with desolvation (175) was carried out. The amount of oxygen added to the Ar aerosol carrier gas (and, in some cases, the plasma gas) (174) is, however, critical since enough must be added for complete carbon combustion without inducing the formation of interfering oxide polyatomic ions (unless, for example, the determination of P is done using PO+ at m/z 47) (52) or reducing sensitivity (174). The MS interface was shown to either raise or lower ion number densities in the ICP, depending on the aerosol carrier gas flow rate (176). The number density of Ca ions decreased along with the electron number density in the presence of the MS interface, which suggests that ionization of Ca occurs through electron impact. In contrast, Sr ion density increased in the presence of the MS interface. In any case, the presence of 0.01 M Li, Cu, and Zn matrix elements reduced the ion number densities of the 0.1 mM Ca and Sr analytes, especially at high carrier gas flow rate, i.e., under conditions that were not robust, whether the MS interface was present or not. The suppression increased in the matrix order of Zn, Cu, and Li (176). Although this was not discussed in the paper, there appears to be a correlation with the first IP of the matrix element since Zn and Li have, respectively, the highest and lowest IPs for this group. So, the increased suppression at a fixed height in the ICP may simply be the result of a greater shift in atom-ion equilibrium, which in turn shifted the IRZ. Measurements at different heights would be needed to verify this possibility. Chemometrics. A chemometric technique called common analyte internal standardization was shown to be more generally applicable than internal standardization since, in contrast to the latter, it depends on the difference in behavior between the analyte and the internal reference element (177). Yet, it can compensate for both drift and nonspectroscopic interferences. If only drift is problematic, then one internal reference element must be added to all samples, blanks, and standards. Unlike internal standardization, there is no requirement that this element be matched in ionization potential or mass to those of the analytes. The only requirement (in addition, of course, to not being present in the samples) is that its signal be very susceptible to drift. The standards, which are measured at the beginning, middle, and end of each run, are used to establish a drift correction equation in the form of (I0a/I a) ) K[(I0i - I i)/I i]+1, where I0 and I are respectively the initial and drift-affected measurement of the analyte (a) and the internal reference element (i). The analyte intensities measured in all samples, blanks, and standards are then corrected using this equation, where different values of K can be obtained for different analytes. To correct for both drift and nonspectroscopic interferences, a second internal reference element must be added to all samples, blanks, and standards, and additional standards containing varying concentrations of one
easily ionized matrix element must be aspirated immediately following the matrix-free standards. The intensities of the latter are subtracted from the former to isolate the change in signal that is due to the matrix and then enable the derivation of a matrix correction equation of a form that is similar to that of the drift correction equation. For this approach to be effective, the second internal reference element must behave differently from the first internal standard element (i.e., should have a different IP and mass). The approach was successfully applied to the analysis of river and underground waters. It will be interesting to see if it is as effective with more complex matrixes that are not constituted mainly of easily ionized elements. ISOTOPE RATIOS Many fields, such as the measurement of isotope variation in nature or tracer experiments in biological or medical studies, require the precise and accurate determination of isotope ratios. Similarly, the efficiency of internal standardization and isotope dilution analysis directly depends on the quality of the isotope ratios measured. The accuracy of isotope ratios is also of outmost importance when trying to determine the equilibrium dissociation temperature in ICPMS. Indeed, several methods involve the measurement of several oxide ratios (MO+/M+). The most reliable of these methods appeared to be a modified Boltzmann plot, which takes into account the rotational constants and degeneracies of the MO+ ions, as well as the partition functions of the atomic and ionic species involved in their dissociation equilibrium (178). It yielded a dissociation temperature of 7172 K. The various ICPMS instruments that are available for the measurement of isotope ratios were extensively reviewed as well as their capabilities and limitations (such as mass discrimination and detector dead time) (179). A product review emphasized the new doors that multiple-collector ICPMS opened since it indeed allows very precise isotope ratio measurements of heavy stable isotopes (such as transition elements) that are inaccessible by either isotope ratio MS or thermal ionization MS (TIMS) (180). Furthermore, sample preparation of solid samples is simple compared to that required for TIMS (178). Different strategies were specifically compared for the determination of Fe isotope ratios in human serum (i.e., in the presence of Ca) by ICPMS (181). These included using cool plasma conditions, desolvation, a DRC or DF sector field MS at a mass resolution of 3000. With a cool plasma, Ar-containing polyatomic ions were reduced but Ca-containing polyatomic ions interfered more than under standard (hot) plasma conditions. Desolvation was more efficient than a cool plasma at reducing interferences at m/z 57 and 58 but less efficient for m/z 54 and 56. On the other hand, the interference suppression with desolvation was unaffected by the presence of Ca. Although the precision of Fe isotope ratios obtained with a DRC was better than that with a DF instrument for aqueous solutions, the accuracy and precision were substantially degraded for human serum as a result of matrixinduced mass discrimination. Even with matrix-matched blanks and standards, only 54Fe/56Fe could be appropriately determined in human serum, in contrast to the DF instrument, which allowed the accurate determination of all Fe ratios in this matrix without any requirement for matrix-matching. Mass discrimination with
a DRC was in fact demonstrated to be dependent on a number of parameters, such as the collision or reaction gas flow rates and band-pass setting, in addition to matrix composition (182). Unless the analyte is separated from matrix components, a matrixmatched isotopic standard is required to correct measured ratios for mass discrimination as internal correction was found to be inadequate. A difference in mass discrimination was found between instruments having a hexapole collision cell instead of a set of electrostatic ion focusing lenses between the sampling interface and the quadrupole (183). The higher mass discrimination at low mass but lower mass bias at high mass exhibited with collision cell was attributed to the absence of lenses, which may be an additional source of mass bias. In any case, the use of a collision gas did not significantly change the expected exponential decay of mass bias with mass; i.e., no additional mass bias was observed. When simultaneous detection instruments are used, isotopic ratios can be measured with a precision that is limited only by Poisson counting statistics and can therefore be improved by simply increasing the integration time (51). Even for transient signals, any signal fluctuations, which were shown to be correlated for isotopes of a given element, simply cancel upon calculating the ratio (51). However, because of the lower sensitivity of TOF instruments compared to quadrupole-based ones, the precision of isotope ratios from transient signals degrades as the analyte concentration decreases to the point where a quadrupole instrument could yield ratios of better precision than TOF even when 25 isotopes were measured (184). Only when the full width at half-maximum was less than 8 s or when more than 15 isotopes were to be measured did a TOF mass analyzer provide isotope ratios with better precision. However, the determination of isotope ratios from transient signals is not as simple and straightforward as from steady-state signals. For instance, the determination of peak height and peak area depends on peak shape, background level, and the extent of resolution from adjacent peaks. Yet, the average of point-to-point ratios calculated throughout a flow injection peak had similar or better precision than those obtained from taking the ratio of heights or areas (185). In fact, an improvement in precision resulted from averaging the ratios corresponding only to the peak plateau that was obtained when injections were made into air or surrounded by air plugs, dispersion then being essentially eliminated. Even when simultaneous detection was done with TOF-ICPMS, the best precision on isotope ratios measured in transient mode were obtained using signals from the peak apex, especially when a steady state was temporarily reached (186). The integration time should therefore be as long as possible while a good peak definition is maintained with at least five data points over the peak apex. Although this approach was suitable for the detection of FI peaks, it was found to yield less accurate and precise ratios than trapezium area integration for GC transient signals. In fact, trapezium area integration of the peak center was found to be more precise than full peak area integration when peak tailing, inaccurate baseline subtraction (as a result of difficulty in assigning the start and end of the peak), or clear isotope fractionation across the peak (i.e., where the heavier isotope preferentially eluted Analytical Chemistry, Vol. 76, No. 12, June 15, 2004
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ahead of the light isotope) would introduce an error (187). In any case, more precise ratios were systematically obtained with MC than TOF instruments (186, 188). Although both allow simultaneous detection over a narrow (with MC) or wide (TOF) mass range, their associated data acquisition systems are quite different. Indeed, the former uses a fixed number of Faraday cups to measure isotopes of, usually, a single element, whereas the latter uses an electron multiplier in either counting or analog mode. Yet, the gain provided by an electron multiplier is neither uniform nor consistent with time, which will limit the precision attainable even if it is used, like Faraday cups, in analog mode (189). Furthermore, only one count can be processed at a time in the counting mode, which, to avoid pulse pileup, means that the average time between counts should be at least 10 times the dead time of the detection electronics. Finally, the duty cycle is essentially 100% with MC but typically only 5-10% with TOF (albeit independent of the number of channels) due to the ion beam modulation that is required for the latter. Therefore, because the MC system can acquire a much greater amount of data simultaneously, it will be superior to TOF for the measurement of high-precision isotope ratios. Moreover, simultaneous detection of isotope ratios (for that or other purposes, such as internal standardization or isotope dilution analysis) overcomes multiplicative (flicker) noise and allows precision limited only by Poisson counting statistics (as mentioned earlier). This precision can only be achieved with sequential instruments if the scanning rate exceeds the predominant ICPMS noise frequencies. Otherwise, precision in such sequential systems will be limited to 0.5-5.0% RSD. Furthermore, because of their 100% duty cycle, MC instruments offer increased effective sensitivity for multielement analysis, which reduces the analysis time required to reach a certain signal-to-noise level and, in turn, sample consumption (188). Of course, the accuracy of isotope ratios also depends on any mass bias correction. Although the determination of this correction factor may be straightforward with continuous nebulization, using a standard of known isotopic composition, it may become complicated for transient signals. For example, the mass bias determined using an Sb solution was found to be significantly different from that measured using a gas standard (187). This observation therefore precluded the use of continuous nebulization with GC (where the GC effluent was merged with the wet aerosol) and necessitated bracketing each sample with a gas standard. In fact, even with continuous nebulization, the accurate determination of mass bias will depend on the correction expression used. Indeed, the deviation of a particular isotope ratio from the expected value was demonstrated to change with m/z, which means that current correction expressions (based on power, linear, and exponential mass bias) are fundamentally incorrect since they all assume that the mass bias is constant across the mass range (190). Instead, an instrument response function, which is expected to be characteristic of a given instrument and different from that of others, should be established over a wide mass range. In this respect, TOF instruments, which can acquire data for a large mass range essentially simultaneously, should allow this determination with great precision, which should in turn allow a detailed mass bias model to be constructed. Since the mass bias 3412
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was shown to vary with the matrix (including its concentration), the use of internal standards is recommended for this purpose. The most accurate isotope ratios may also require a correction for dead time (i.e., the minimum time interval between the arrival of two ions so that they may be distinguished by an electron multiplier) if a detector is used in the counting mode. The various methods that are available to determine the dead time were reviewed and demonstrated to all be unbiased, although some of them yielded a more precise dead time, which through error propagation would result in ratios of better precision (191). It may even be wise to check the instrument software that is used to integrate the mass spectral peaks prior to calculating their ratios. Indeed, one study reported an improvement in the isotope ratio measurement repeatability at medium mass resolution by up to 1 order of magnitude when the raw data were processed off-line using spreadsheet calculations (192). This improved precision was also 2.6 times more stable over a 45-min period than that achieved using the instrument software. With MC instruments, the measured ion beam intensities must be corrected for the zero offset of each Faraday amplifier, which is typically done by taking integrations at (0.5 u from the peak mass and subtracting the mean of these measurements from the raw peak intensity before ratio calculation. However, when there is tailing from an adjacent peak, the extent of which depends on the abundance sensitivity of the instrument, an overcorrection of such tailing is then concurrently done since tailing is not linear but concave with mass (193). In these cases, the automatic correction should be disabled to allow its determination. This correction can be obtained through a measurement of the tail profile done using a monoisotopic element (in an analyte-free solution) within the mass range of the isotope ratios to be determined. In any case, the precision of ratios measured on MC instruments can be improved by using time-resolved analysis, where individual signal intensities are recorded every 0.2 s and the isotopic ratios calculated from the intensity data obtained during their plateau (194). Isotope Dilution. A detailed uncertainty budget was described for trace analysis by ID-ICPMS (195). This budget includes corrections for dead time, background subtraction, isobaric interference (using an interference-free isotope of the interfering element), mass discrimination, blank, and even air buoyancy. With a correct consideration of the correlation, double counting of separate contributions to the uncertainty budget was avoided. In fact, the contributions of the isotopic abundances of spike and samples, the dead time, and the air buoyancy were only minor. Nonetheless, this should be verified for each application. Indeed, the densities of the samples and the references, which are linked to the air density, need to be known for metrological mass determinations, which therefore require a correction for air buoyancy (196). In any case, apart from accurate mass measurement, the most important contribution to the combined measurement uncertainty in the determination of Pb in wines by ID using MC-ICPMS was the isotope amount ratio of the mixture between sample and enriched isotopic spike. Sample pretreatment, which in this case involved sample digestion and matrix separation, was the third most important contributor.
INSTRUMENT PERFORMANCE A new compact ICPMS instrument was introduced by Varian, which features an innovative 90° reflecting ion optics (197). It involves a hollow ion mirror through which photons and neutrons pass, thereby reducing the background and minimizing contamination of the ion optics. The parabolic electrostatic field of this ion mirror not only reflects ions at a right angle from the incoming ion beam but it concurrently focuses them, hence providing high sensitivity (over 109 counts s-1 mg-1 L). This is achieved while keeping the CeO+/Ce+ oxide level below 3%. The instrument can also be operated in “normal-sensitivity mode” instead of the “highsensitivity mode” to allow an extension of the linear dynamic range up to several hundred milligrams per liter. Hitachi introduced, in Japan only, an instrument combining an ICP with an ion trap. Ions extracted from the ICP are guided by Einzel lenses through a double cylindrical electrostatic ion guide to a 90° deflector and the ion trap. The latter was said to be better than a collision cell, which is really an open trap, since it can be used to trap the analyte and get rid of everything else. Changing the trapping time can expand its dynamic range (5-7 orders of magnitude), which is limited compared to that observed on other existing instruments (198). A source of memory effect, namely, the revolatilization of analyte deposited downstream of the ion sampling interface, could be alleviated by applying a small retarding voltage on either an ion lens or the quadrupole without affecting ion transmission of analyte ions from the ICP. Indeed, background ions were shown to have an energy significantly lower than that of ions originating from the plasma (199). An approach involving two nebulizer/spray chamber systems whose exits are connected through a Y adapter at the base of the ICP torch was proposed to perform on-line standard additions (200). Although it eliminates the preliminary spiking of sample solutions prior to analysis, the relative sample introduction efficiency of the two systems must be determined and monitored. The accuracy of the approach, which was equivalent to that achieved by the regular method of standard additions, was only demonstrated by the analysis of a 35 mM Na synthetic sample. It is rather ironic that, although the authors extol the great simplicity of their approach, they did not apply it to a single real sample. Perhaps this is the result of a condensation problem in the tubing and adapter connecting the spray chambers to the torch. It has certainly been this author’s experience with two different instruments that, despite many claims that small droplets are transported too quickly to have time to settle, condensation occurred in any transfer line between a spray chamber and the torch. This problem would indeed be expected to worsen when using an adapter in addition to transfer lines. A simpler approach was used to implement both internal standardization and the method of standard additions on-line with ICPMS. On-line merging of the sample with a toluene solution containing an internal standard and increasing analyte amounts was indeed done by Duyck et al. prior to nebulization for the successful analysis of crude oil and its fractions (175). In any case, the direct analysis of complex samples is better performed following a multivariate optimization of the ICPMS operating conditions. This can be accomplished using different multivariate statistical methods, such as factorial design, principal component
analysis, and cluster analysis (174). The conditions for the analysis of organic solutions are indeed completely different from those for aqueous solutions. Diane Beauchemin received a B.Sc. in chemistry from the Universite´ de Montre´ al and stayed on to obtain a Ph.D. in analytical chemistry at the end of 1984 (under the supervision of Joseph Hubert). She then worked as a Research Associate in the Chemistry Division (now the Institute for National Measurement Standards) of the National Research Council of Canada, in collaboration with Jim W. McLaren. She moved to Queen’s in July 1988 where she was recently promoted to Full Professor (July 1, 2001). She is interested in all aspects of ICPMS, from fundamental studies on matrix effects and mixed-gas plasma to environmental and biomedical applications (including chemical speciation).
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